MEIOSIS

1
CHAPTER 13
MEIOSIS SEXUAL LIFE CYCLES
2
INTRODUCTION TO HEREDITY

• HEREDITY CONTINUITY OF BIOLOGICAL TRAITS FROM
  ONE GENERATION TO THE NEXT
• VARIATION INHERITED DIFFERENCES AMONG
  INDIVIDUALS OF THE SAME SPECIES
• GENETICS THE STUDY OF HEREDITY AND VARIATION

3
HEREDITY

• OFFPRING ACQUIRE GENES FROM PARENTS BY INHERITING
  CHROMOSOMES
• DNA TYPE OF NUCLEIC ACID
• GENES UNITS OF HEREDITARY INFO THAT ARE MADE OF
  DNA AND ARE LOCATED ON CHROMOSOMES

4
HEREDITY

• THE ACTUAL TRANSMISSION OF GENES FROM PARENTS TO
  OFFSPRING DEPENDS ON THE BEHAVIOR OF CHROMOSOMES
• CHROMOSOMES ORGANIZATIONAL UNIT OF HEREDITY
  MATERIAL IN THE NUCLEUS OF EUKARYOTIC ORGANISMS
• LOCUS SPECIFIC LOCATION ON A CHROMOSOME THAT
  CONTAINS A GENE

5
THE ROLE OF MEIOSIS IN SEXUAL LIFE CYCLES

• FERTILIZATION AND MEIOSIS ALTERNATE IN SEXUAL
  LIFE CYCLES
• ALTERNATION BETWEEN HAPLOID (n) AND DIPLOID (2n)
  CONDITION
• SOMATIC CELLS - CONTAIN 46 CHROMOSOMES
• KARYOTYPE A DISPLAY OF AN INDIVIDUALS
  SOMATIC-CELL METAPHASE CHROMOSOMES

6
SEXUAL LIFE CYCLES

• HOMOLOGOUS CHROMOSOMES A PAIR OF CHROMOSOMES
  THAT HAVE THE SAME SIZE, CENTROMERE POSITION AND
  STAINING PATTERN
• AUTOSOME - A CHROMOSOME THAT IS NOT A SEX
  CHROMOSOME (22 PAIRS IN HUMANS)
• SEX CHROMOSOMES IT IS THE 23RD CHROMOSOME PAIR
  IN HUMANS
• FEMALES (XX)
• MALES (XY)

7
ALTERNATION OF GENERATIONS

• DIPLOID CELLS CONTAIN TWO SETS OF CHROMOSOMES
  (2n)ALL AUTOSOMES
• HAPLOID CELLS CONTAIN ONE SET OF
  CHROMOSOMES(n) GAMETES
• GAMETE SEX CELLS SPERM AND OVA

8
FERTILIZATION

• THE DIPLOID NUMBER IS RESTORED DURING
  FERTILIZATION, THE UNION OF TWO GAMETES
• ZYGOTE A DIPLOID CELL FORMED FROM THE UNION OF
  TWO HAPLOID GAMETES

9
SEXUAL LIFE CYCLE VIDEO
10
ASEXUAL LIFE CYCLE VIDEO
11
MEIOSIS A CLOSER LOOK

• MEIOSIS AND SEXUAL REPRODUCTION GREATLY
  CONTRIBUTE TO GENETIC VARIATION AMONG OFFSPRING
• MEIOSIS INCLUDES STEPS THAT CLOSELY RESEMBLE
  MITOSIS, SO YOU ALREADY KNOW A LOT OF THIS!

12
MEIOSIS V. MITOSIS

• LIKE MITOSIS, MEIOSIS IS PRECEDED BY REPLICATION
  OF CHROMOSOMES
• MEIOSIS DIFFERS IN THAT THIS SINGLE REPLICATION
  IS FOLLOWED BY TWO CONSECUTIVE CELL DIVISIONS
• FOUR DAUGHTER CELLS ARE FORMED DURING MEIOSIS
• THE DAUGHTER CELLS ARE HAPLOID, THEY ONLY HAVE
  HALF THE NUMBER OF CHROMOSOMES AS THE ORIGINAL
  CELL

13
MEIOSIS-INTERPHASE I

• CHROMOSOMES REPLICATE
• EACH DUPLICATED CHROMOSOME CONSISTS OF TWO
  IDENTICAL SISTER CHROMATIDS ATTACHED AT THEIR
  CENTROMERES
• CENTRIOLE PAIRS IN ANIMAL CELLS ALSO REPLICATE
  INTO TWO PAIRS

14
MEIOSIS I

• THIS CELL DIVISION SEGREGATES THE TWO CHROMOSOMES
  OF EACH HOMOLOGOUS PAIR AND REDUCES THE
  CHROMOSOME NUMBER BY ONE-HALF. IT INCLUDES THE
  FOLLOWING FOUR PHASES
• PROPHASE I, METAPHASE I, ANAPHASE I, TELOPHASE I

15
PROPHASE I

• THIS PHASE TAKES UP 90 OF THE TIME REQUIRED FOR
  MEIOSIS
• CHROMOSOMES CONDENSE
• SYNAPSIS OCCURS HOMOLOGOUS CHROMOSOMES COME
  TOGETHER AS PAIRS
• SINCE EACH CHROMOSOME HAS TWO CHROMATIDS, EACH
  HOMOLOGOUS PAIR IN SYNAPSIS APPEARS AS A COMPLEX
  OF FOUR CHROMATIDS, OR A TETRAD

16
PROPHASE I (CONT)

• IN EACH TETRAD, SISTER CHROMATIDS OF THE SAME
  CHROMOSOME ARE ATTACHED AT THEIR CENTROMERES.
  NONSISTER CHROMATIDS ARE LINKED BY X-SHAPED
  CHAISMATA, SITES WHERE HOMOLOGOUS STRAND EXCHANGE
  OR CROSSING OVER OCCURS.
• CROSSING OVER IS VERY IMPORTANT. THIS EXCHANGE OF
  GENETIC MATERIALS ALLOWS FOR GENETIC VARIATION
  AMONG OFFSPRING

17
PROPHASE I VIDEO
18
METAPHASE I

• TETRADS ARE ALIGNED ON THE METAPHASE PLATE
• EACH SYNAPTIC PAIR IS ALIGNED SO THAT CENTROMERES
  OF HOMOLOGUES POINT TOWARD OPPOSITE POLES
• EACH HOMOLOGUE IS ATTACHED TO KINETOCHORE
  MICROTUBULES EMERGING FROM THE POLE IT FACES, SO
  THAT THE TWO HOMOLOGUES SEPARATE IN ANAPHASE AND
  MOVE TOWARD OPPOSITE POLES

19
METAPHASE I VIDEO
20
ANAPHASE I

• HOMOLOGUES SEPARATE AND ARE MOVED TOWARD THE
  POLES BY THE SPINDLE
• SISTER CHROMATIDS REMAIN ATTACHED AT THEIR
  CENTROMERES AND MOVE AS A UNIT TOWARD THE SAME
  POLE, WHILE THE HOMOLOGUE MOVES TO OTHER POLE
• THIS DIFFERS FROM MITOSIS DURING WHICH
  CHROMOSOMES LINE UP INDIVIDUALLY ON METAPHASE
  PLATE, AND SISTER CHROMATIDS ARE SPLIT APART

21
ANAPHASE I VIDEO
22
TELOPHASE I AND CYTOKINESIS

• EACH POLE NOW HAS A HAPLOID SET OF CHROMOSOMES
  THAT ARE STILL MADE OF TWO SISTER CHROMATIDS
  ATTACHED AT THE CENTROMERE
• CYTOKINESIS OCCURS, FORMING TWO HAPLOID DAUGHTER
  CELLS. CLEAVAGE FURROWS FORM IN ANIMAL CELLS

23
TELOPHASE I VIDEO
24
MEIOSIS II

• THERE IS NO INTERPHASE II. THE REASON IS THAT
  THE DNA WILL NOT BE DUPLICATED AGAIN BEFORE THE
  SECOND CELL DIVISION
• THIS SECOND MEIOTIC DIVISION SEPARATES SISTER
  CHROMATIDS OF EACH CHROMOSOME

25
PROPHASE II

• SPINDLE APPARATUS FORMS AND CHROMOSOMES MOVE
  TOWARD THE METAPHASE II PLATE

26
METAPHASE II

• CHROMOSOMES ALIGN SINGLY ON THE METAPHASE PLATE
• KINETOCHORES OF SISTER CHROMATIDS POINT TOWARD
  OPPOSITE POLES

27
ANAPHASE II

• CENTROMERES OF SISTER CHROMATIDS SEPARATE
• SISTER CHROMATIDS OF EACH PAIR (NOW INDIVIDUAL
  CHROMOSOMES) MOVE TOWARD OPPOSITE POLES OF THE
  CELL

28
TELOPHASE II

• NUCLEI FORM AT OPPOSITE POLES OF THE CELL
• CYTOKINESIS OCCURS PRODUCING FOUR HAPLOID
  DAUGHTER CELLS

29
MEIOSIS II VIDEO
30
KEY DIFFERENCES MEIOSIS AND MITOSIS

• MEIOSIS IS A REDUCTION DIVISION. CELLS PRODUCED
  BY MITOSIS HAVE THE SAME AS THE ORIGINAL CELL,
  WHEREAS CELLS MADE BY MEIOSIS HAVE HALF THE OF
  CHROMOSOMES
• MEIOSIS CREATES GENETIC VARIATION DUE TO CROSSING
  OVER, WHICH IS ABSENT IN MITOSIS
• MEIOSIS IS TWO SUCCESSIVE NUCLEAR DIVISIONS,
  WHEREAS MITOSIS IS JUST ONE DIVISION

31
COMPARISONMEIOSIS I AND MITOSIS
MEIOSIS I
MITOSIS

• PROPHASE SYNAPSIS OCCURS TO FORM TETRADS
• METAPHASE HOMOLOGOUS PAIRS (TETRADS) ALIGN
• ANAPHASE PAIRS OF CHROMOSOMES SEPARATED.
  CENTROMERE DO NOT DIVIDE, SISTER CHROMATIDS STAY
  TOGETHER, MOVE TO SAME POLE

• PROPHASE NO SYNAPSIS OR CROSSING OVER
• METAPHASE INDIVIDUAL CHROMOSOMES ALIGN
• ANAPHASE SISTER CHROMATIDS OF INDIVIDUAL
  CHROMOSOMES SEPARATE AND MOVE TO OPPOSITE POLES

32
ORIGINS OF GENETIC VARIATION

• SEXUAL REPRODUCTION CONTRIBUTES TO GENETIC
  VARIATION BY
• INDEPENDENT ASSORTMENT OF CHROMOSOMES
• CROSSING OVER IN MEIOSIS I
• RANDOM FUSION OF GAMETES DURING FERTILIZATION

33
INDEPENDENT ASSORTMENT

• DURING METAPHASE I, EACH HOMOLOGOUS PAIR OF
  CHROMOSOMES ALIGNS. EACH PAIR CONSISTS OF ONE
  MATERNAL AND ONE PATERNAL CHROMOSOME
• THERE IS A 50-50 CHANCE THAT A DAUGHTER CELL WILL
  RECEIVE EITHER MATERNAL OR PATERNAL CHROMOSOME
• EACH HOMOLOGOUS PAIR ORIENTS INDEPENDENTLY

34
INDEPENDENT ASSORT (CONT)

• SINCE EACH PAIR ASSORTS INDEPENDENTLY, THE
  PROCESS PRODUCES 2n POSSIBLE COMBINATIONS OF
  MATERNAL AND PATERNAL CHROMOSOMES IN GAMETES,
  WHERE n IS THE HAPLOID
• IN HUMANS, THE POSSIBLE COMBINATIONS WOULD BE
  223, OR ABOUT 8 MILLION
• THUS, EACH HUMAN GAMETE CONTAINS ONE OF EIGHT
  MILLION POSSIBLE ASSORTMENTS OF CHROMOSOMES
  INHERITED FROM BOTH PARENTS

35
INDEPENDENT ASSORTMENT
36
CROSSING OVER

• OCCURS DURING PROPHASE I, CHAISMATA BECOME
  VISIBLE AT PLACES WHERE THE EXCHANGE OCCURS
• PRODUCES CHROMOSOMES THAT CONTAIN GENES FROM BOTH
  PARENTS
• IN HUMANS, THERE IS AN AVERAGE OF TWO OR THREE
  CROSSOVER PER CHROMOSOME PAIR
• SYNAPSIS IS PRECISE, HOMOLOGUES ALIGN GENE BY GENE

37
CROSSING OVER
38
RANDOM FERTILIZATION

• IN HUMANS, WHEN INDIVIDUAL OVUM REPRESENTATIVE OF
  ONE OF EIGHT MILLION POSSIBLE COMBINATIONS IS
  FERTILIZED BY A SPERM CELL WITH THE SAME
  POSSIBILITIES, THE RESULTING ZYGOTE CAN HAVE ONE
  OF 64 TRILLION POSSIBLE DIPLOID COMBINATIONS